The foundation for a factory-built house is the first concrete pour on the plot and the first place where such a house differs from one built on site: the elements do not give. If the foundation is two centimetres off or not level, the walls do not fit, and correcting that on site is expensive. That is why we talk about the house foundation so much before the first element goes into production.

Below are six things that decide which foundation a CLT house needs, what shapes its cost and how to write that line into your budget.

Why does the foundation of a CLT house need to be so precise?

CLT elements are manufactured in the factory according to detailed production drawings for each individual house. The walls, floors and other load-bearing elements arrive on site already cut to their final dimensions, with their positions defined to millimetre accuracy.

The same level of precision is therefore required from the foundation. The exact positions of the CLT elements are marked on the slab, and all necessary water and drainage pipes, floor drains, electrical feeds, conduits and other service penetrations must be positioned correctly before the concrete is poured.

These locations cannot simply be moved later. Even an error of a few centimetres can mean that a pipe or penetration ends up inside the wrong wall, outside the intended room or no longer aligns with the next stage of construction.

For this reason, a surveyor sets out the building and the relevant dimensions, axes, levels and project-defined positions are carefully checked. A precise foundation provides the correct starting point for CLT installation and for every construction stage that follows.

We look at technical service penetrations in more detail below.

What type of foundation do we use at Happy Home?

Our standard solution is an insulated reinforced-concrete slab foundation.

Its basic build-up is similar across our houses and, depending on the project, includes:

• a prepared and compacted sub-base;
• insulated plinth elements;
• EPS insulation;
• a reinforced concrete slab with the necessary local thickening;
• perimeter insulation around the building;
• project-specific service penetrations and technical preparations.

Special composite boards are also used beneath windows to help minimise thermal bridging.

The exact structural design of the foundation is calculated for the particular house and site conditions, while the underlying technical solution remains largely consistent across Happy Home projects.

Groundworks depend on the plot

Before the foundation is built, the topsoil is removed from the building area and the necessary excavation works are carried out.

Where suitable, the excavated topsoil can be left on the plot at the client’s request and later reused for landscaping or adjusting ground levels. If the plot is small, the volume of excavated material is large or the client simply prefers it, we can also arrange for the soil to be removed and disposed of as an additional service.

The area beneath the foundation is then filled and compacted using the materials specified in the design. A typical Happy Home quotation includes up to 50 cm of fill beneath the foundation.

If substantially deeper excavation is required to reach a suitable load-bearing layer, or if bedrock requires hydraulic breaking, additional works and material may be needed.

Situations that require a significantly different approach are relatively uncommon, but where necessary the solution and any associated cost are reviewed separately for the specific site.

Technical systems are already planned into the foundation

A foundation is not simply the structural base of the house. Before the concrete is poured, many of the services that pass through the floor or foundation must already be precisely defined.

Water and drainage pipes, floor drains, electrical and low-voltage conduits and other required penetrations are positioned according to the relevant technical drawings.

The building is provided with the required earthing system, and a temporary electrical distribution board is installed for the construction period.

For example, if the kitchen extractor is designed with an exhaust duct running through the floor, the duct is already installed at this stage from the kitchen location through the foundation to the exterior of the building.

These solutions therefore need to be resolved before the foundation is poured. This is one reason why the architectural, interior and technical designs need to be coordinated with one another early in the process.

When is radon protection needed?

Radon is a naturally occurring radioactive gas that originates in the ground and can enter buildings through structures in contact with the soil. It cannot be detected by smell or colour. Some areas of Estonia have a higher natural radon risk, particularly in Northern Estonia, although higher-risk zones also occur elsewhere.

A regional indication of radon risk can be checked on the Estonian Geological Survey’s radon risk map, while a site-specific survey provides more accurate information for an individual plot.

Radon protection is not installed automatically in every Happy Home foundation. The need for it is agreed with the client based on the site and the information available.

If a radon survey indicates an elevated risk, or if the client prefers to add protection as a precaution without commissioning a survey, a radon membrane can be installed between the insulation layers of the foundation.

Where the radon risk is higher, the design may also include additional measures such as radon sumps, extraction pipework or provision for a radon fan. These solutions are designed and priced separately according to the specific project.

Any necessary measures should ideally be agreed before the foundation is completed, as adding them later is considerably more difficult.

How is rainwater managed?

Rainwater drainage is shown as a separate item in the Happy Home quotation and is installed for a large proportion of our houses.

The required rainwater gullies and pipework are installed around the house so that water collected from the roof is directed safely away from the building according to the solution designed for the particular site.

The final discharge point depends on the plot, local conditions and the project. Depending on the site, rainwater may be directed into a stormwater network, infiltrated on the property or collected in a separate rainwater tank.

Rainwater management should be considered together with the site levels, foundation and overall external works from the design stage onwards.

When is perimeter drainage needed?

Perimeter drainage is different from the rainwater drainage system.

On some projects, drainage is required because of the soil conditions, site levels, groundwater or another technical consideration. In other cases, the client may choose to add it as an additional precaution.

Its purpose is to collect excess water in the soil and direct it away from the foundation. A typical system uses a perforated drainage pipe surrounded by free-draining gravel and geotextile, with the pipework laid to the required fall so that water can flow to the intended discharge point.

The exact layers, pipe locations and discharge solution are always determined according to the specific plot.

If perimeter drainage is included, it makes sense to install it together with the foundation and other groundworks. Adding the same system later, once the house, terraces and landscaping are complete, is considerably more complicated.

We do not compromise on foundation quality

The foundation is difficult to correct later, and its accuracy directly affects CLT installation and every stage that follows. For this reason, we use a carefully developed foundation solution and do not compromise on the quality of materials, workmanship or execution.

Some clients choose to commission the foundation from their own contractor or organise this part of the construction themselves. That is entirely possible, but in such cases we carry out a thorough inspection before CLT installation, checking the foundation dimensions, levels, element positions and service penetrations.

We have encountered cases where inaccuracies in foundations built by third parties become apparent during preparation for CLT installation or during the installation itself – for example, where the structural dimensions or service penetrations do not correspond with the project. Some issues may also only become visible during later construction stages.

A non-standard foundation solution is rarely required, but certain plots or projects may call for one. This may involve a different type of foundation, unusually extensive groundworks, difficult rock excavation or another technical solution. In these cases, we discuss the options with the client and price the additional work according to the project.

The foundation sets the level of precision for the entire CLT house

Most of the foundation disappears from view once the house is complete, but its quality has a direct impact on everything that is built above it.

For a CLT house, a good foundation means more than sufficient structural capacity. It must have the correct dimensions and levels, be accurately set out, properly insulated and contain all required service penetrations and connections in exactly the right locations.

That is why we treat the foundation as a complete technical solution rather than simply a concrete slab. When the foundation is properly designed and precisely built from the outset, the accuracy of the factory-made CLT structure can continue seamlessly on site.